SDHC is just pretty much a standard for SD cards of any flavor (regular, mini, and micro) that have capacities larger than 2GB. Practically, the only difference between SD cards 2Gb and smaller and those that are bigger is the file system used to format them. SD cards 2Gb and lower are formatted using FAT16 file system. The problem is that FAT16 file format has limitation of maximum volume size of 2GB. In order to have an SD card with capacity larger than 2GB you need to format it with FAT32 file system. The problem with that is that the current S60 devices do not talk FAT32 file system, only FAT16 - that's why they are not able to deal with cards bigger than 2GB. Theoretically the capability to address the FAT32 file system should be just a software issue and Nokia should be able to "write it" into firmware but then again I do not know about the Symbian architecture so I'm not 100% on that - might be the Symbian OS limitation in which case firmware is not going to be able to add this capability. 4GB microSD cards have already been announced by at least two major manufcturers and they already doing testing - their availibility is literrally 1 or 2 months away. J and R store in NYC is already taking preorders and I have put my e-mail on the "notify when available" list available on their site. They are almost upon us. The question is if Nokia can and will move to add the compatibilty to their existing and upcoming phones.
And again the only real difference between SD and SDHC cards, whether they are regular, mini, or micro, is the file system used to format them cards bigger than 2GB require FAT32 since FAT16 used up to that point has a 2GB single volume limitation. By the way, FAT32 format allows for volumes up to 32GB in size.
Sorry for the little detour but there seems to be a lot of confusion around about SD and SDHC so I though I would try to clarify a bit.